In drilling a deep oil well, it is necessary to perform a survey. A survey provides data which is converted into a three-dimensional map of the location of the well. While the well may be vertical at the surface where the well begins, it typically will be deviated from a vertical line. Indeed, with the advent of modern steering tools, it is easy to direct a well in lateral directions. This is more and more common in light of many circumstances. At offshore locations, it is not uncommon to erect a single platform in the water and drill 32 wells from that single platform into a producing formation. All 32 wells are positioned through a common 4.times.8 template located under the platform. The wells will typically be parallel for a few hundred feet and then will deviate out into several directions. A few of the wells are approximately vertical while another set of the wells will deviate laterally by a few hundred feet, but the greater number of the wells deviate laterally by several thousand feet. They all eventually reach the total depth for the formation, chosen here for purposes of example, at 10,000 feet. It is therefore necessary to make dynamic surveys while drilling to locate the position in space of each well and to direct continued drilling so that each well actually bottoms at the desired point in the producing formation. In land drilling situations, a number of wells have been drilled in what is sometimes called the Austin chalk. The Austin chalk is a very difficult formation in that it is tight producing zone. The Austin chalk is typically located at about 8,000 feet. A vertical well will pass quickly through the Austin chalk and provide only perhaps 10 to 30 feet of production pay zone. It is, however, now technically feasible to deviate the well from the vertical toward the horizontal so that the well is actually drilled along the formation following its shape, contour and slope. This requires that the well be drilled with an incline in the well which matches the incline or dip of the formation. If, for instance, the formation dips by 30.degree. to the north, the well can be deviated so that a portion of it is inclined at the same angle to the north and is located between the top and bottom faces of the formation to increase the pay zone. In the instances given above, it is necessary to repeatedly provide a survey of the location of the well so that periodic corrections can be made. These corrections are needed so that the well location can be adjusted, so that the well will ultimately terminate at the desired location.
A free fall survey instrument is normally dropped in the drill string. This normally occurs when the drill string is in the well borehole. Whether the drill string is actually being turned or not, the drill string captures the MWD capable survey instrument so that it can provide the necessary confinement to retrieve the free fall survey instrument. Moreover, it is periodically essential to retrieve the entire drill string so that the drill bit can be inspected or replaced. When the drill string is retrieved to the surface, it is normally unthreaded, momentarily stored in the derrick stand by stand, and then repositioned in the well to continue drilling after the drill bit has been changed. This enables recovery of the drill string and recovery of the survey tool which is dropped into the drill string. When the survey tool is dropped into the drill string, it begins a free fall trip, recording data as it falls, and storing that data in an electronic memory device in the survey tool. The stored data is later evaluated once the tool is retrieved and the data can be obtained from the memory in the tool. In that context, it is important that the tool be handled carefully so that jarring of the tool does not damage the tool and perhaps obscure or otherwise interfere with the memory function with the risk of data loss. The free fall survey instrument is exposed to severe shock as it bumps and bangs along the drill string as it falls. If, for instance, it falls in a perfectly vertical well, it will accelerate in velocity until it achieves an equilibrium rate of fall. Typically, the equilibrium is determined by the fluid resistance encountered by the free falling body in the drill pipe. The drill pipe is typically filled with drilling fluid. That normally is a water based additive with heavy materials in it. It is commonly known as mud because it typically includes barites and other weight related minerals which raise the weight of the drilling mud and which make it more resistant to the free falling survey instrument. It is not uncommon for a survey instrument to weight about 100 pounds. If merely dropped in space, and falling 10,000 feet, the streamlined survey instrument will accelerate to perhaps 200 mph; fortunately, the fluid in the drill pipe slows the tool down from that high velocity, but not too much. The free fall survey instrument can be protected by mounting a spring on the bottom of it, and that certainly does reduce the impact when landing at the bottom. Nevertheless, there is still some risk of damage to the survey instrument by impact upon landing. It is not possible to drop the instrument on a cable because that then places some kind of cable or tether in the drill string. That poses a problem because the drill string has to be continuously rotated while mud is pumped down through the drill string for drilling. Generally, the open hole is protected best by continuing mud circulation and continuing drilling so interruption is not desirable.
The present disclosure sets forth an improved structure which is appended to the survey tool. This changes the velocity of the survey instrument when it is dropped in the drill string. Briefly, the present disclosure sets forth an attachment which is placed above the survey instrument. It is attached to it. By use of identical threaded joints, each joint having a fixed length, the buoyancy of the survey instrument is changed so that the velocity of the dropped, free falling survey instrument is changed.
When dropping a weight in free fall, the terminal velocity in a long drop is more or less dependent on the viscosity of the fluid. Working with a given streamlined profile (the survey instrument is relatively streamlined), the device will eventually arrive at a steady state velocity for a particular fluid medium resistance to the instrument. If the drill string were simply filled with air, a very high velocity would be achieved. If the drill string is filled with water, a lesser velocity will be achieved. However, drilling with water is normally not done. Rather, the water is a solvent for additives which convert the water into drilling mud and these, in turn, may change the fluid weight and hence the buoyancy relationship of the survey instrument. In one instance, the drilling mud may be quite light, and in another instance, it may be much heavier. Because of these variations which occur depending on the dynamics of the drilling scheduling, it is not possible to know precisely how much buoyancy change is needed even though the weight of the survey instrument does not change. Working with an example, assume that a survey instrument weighs precisely 100 pounds and is precisely 6 feet in length. The terminal velocity in a 10,000 foot well will differ depending on the drilling fluid in the drill string. The present disclosure is therefore summarized as a system for changing the buoyancy of the survey instrument so that the survey instrument is able to be slowed. This reduces the impact while falling where it bangs against the side of the drill string and it also reduces the impact when it lands at the drill bit. Moreover, this can be changed to accommodate changes in the mud schedule from very light to very heavy drilling muds. Further, the trip of the survey instrument is smoother and stretched out over time; this enables the electronics in the survey instrument to obtain a greater number of measurements because it is in the drill string for a longer interval. The equipment comprises one or more threaded joints affixed to the upper end of the survey tool. Each buoyant joint is lighter than the drilling fluid by a controlled amount. The topmost joint is equipped with a fishing neck for retrieval with a grapple.
An alternative embodiment is also set forth. It utilizes the viscosity of the fluid in the well to retard the fall of the instrument. More specifically, the survey instrument is constructed with a fall retarding structure attached at the top end of the instrument. In one embodiment, this has the form of a spaced, trailing, full width disk or inverted cone. So to speak, it catches the fluid during the fall and creates a drag force on it. The drag force asserted on the free-fall instrument package is dependent on the relative diameter of the retarding device, and the stream lining, or more accurately, the lack of stream lining of the retarding member. In one embodiment, the retarding member is simply a parallel disk which is spaced up from the instrument body. In another instance, it is a conic shape. The conic shape can be formed of thin wall metal so that it is rigid or alternately, it can be formed of a resilient material such as a rubber cone.